In the drilling of wells, for example, in the recovery of oil and gas from subsurface formations, it is very often necessary to depart from the vertical or main well bore by drilling a generally horizontal bore which is initiated along a curved section radius away from the main bore, this curved section radius being customarily referred to as a drainhole. For this purpose, various types of whipstocks have been devised which provide a generally angular guide surface to direct the drill bit in the desired direction or azimuth away from the vertical bore. Ordinarily, whipstock employed for the purpose of drainhole drilling operations may have angled guide surfaces which vary anywhere from 1.degree. to 20.degree. depending upon the degree of curvature desired. For instance, reference is made to U.S. Pat. Nos. 3,398,804 and 3,349,845 granted to me as an inventor and co-inventor, respectively, for a description of drainhole drilling operations employing standard whipstocks. The whipstocks in the methods described in my hereinbefore referred to patents are entirely satisfactory for drilling drainholes where the original or main bore does not deviate from vertical. However, it is very difficult under actual practice to drill a perfectly straight hole; and most oil, gas or water wells of any depth have a significant amount of deviation. It has been found that when the original bore deviates in a lateral or perpendicular direction to any appreciable extent away from the desired drainhole direction of azimuth, under typical drainhole drilling conditions, the standard whipstock will lose its ability to control the direction of drilling and fail to induce curvature development along the proper plane away from the original bore. For instance, if the normal component of deviation is even as low as 1.degree. away from vertical, an error of 10.degree. to 15.degree. in drainhole orientation may be introduced.
It has been proposed in the past to construct whipstocks with a curved guide surface which approximates the curved section radius of the drainhole to be drilled through a lateral wall of the original bore. For example, this was generally suggested in my hereinbefore referred to patents. Others have also made reference to the use of curved or radius whipstocks in drainhole drilling operations. Representative patents are U.S. Pat. Nos. 1,595,922 to O. C. Prindle; 2,158,329 to F. Kinzbach; 2,642,267 and 2,669,428 to J. A. Zublin; 2,709,070 to W. J. Bielstein; 3,116,799 to R. Lemons; and 3,282,355 to J. K. Henderson. However, there has been notably lacking in the past any recognition of the problem of correcting or compensating for lateral deviation of the original hole away from vertical either in the design of the whipstock guide surface or in the method of orientation of the whipstock guide surface with respect to initiation of the curved section radius of drainhole along the proper azimuth.